In the case of electrical energy stores such as batteries, for example, the current charge able to be drawn is an important variable, since it expresses the energy reserve still available before a minimum capacity required of the energy store is undershot. Especially in the field of automotive technology, a precise prediction of the charge able to be drawn is more decisive than the knowledge of the current charge state of the battery defined via the average acid concentration in the lead accumulator, since the latter only provides information about the charge already drawn in relation to the full charge, but not, however, about the amount of charge that is still able to be drawn.
The entire charge still able to be drawn immediately determines the availability of the electrical loads connected to the energy store. The knowledge of the charge able to be drawn may additionally be used for measures of open-loop or closed-loop control technology such as are used, for example, for an energy management system in a vehicle. This makes it possible, for example, to initiate, in time before reaching a minimum charge reserve, consumption-reducing measures such as switching off or dimming less important loads.
A method is described in published European patent document EP-0376967 to determine the charge able to be drawn from an energy store. In this instance, the charge able to be drawn is estimated via empirically ascertained characteristics maps, which are stored in a processing unit, as a function of a constant discharging current, of the battery temperature and of aging effects on the basis of the Peukert formula. To be sure, this makes it possible to ascertain the charge able to be drawn up to a cutoff, which is characterized by the complete discharge of the energy store; however, it is not possible to determine the charge able to be drawn before undershooting a specified minimum terminal voltage or before undershooting a minimum capacity of the energy store. Moreover, determining the charge able to be drawn on the basis of the Peukert formula is relatively imprecise, since different effects influencing the state of the cutoff such as, e.g., an active mass loss at the electrodes due to the ageing of the battery or the formation of ice at the electrodes at low temperatures are not taken into account.
It is therefore an objective of the present invention to provide a device and a method for ascertaining the charge able to be drawn from an energy store, which allow for a very precise determination of the charge able to be drawn before meeting a specified cutoff criterion.